I. Field of the Invention
The present invention relates to a portable apparatus for readily and easily measuring the primary winding to secondary winding current ratio of a current transformer while that transformer is in service.
II. Background Information
Large current transformers, capable of receiving primary current loads on the order of 200 to 12,000 amperes are well known. Such current transformers typically (when utilized at a rating factor of one) have a primary to secondary current ratio from 200:5 to 400:5 for current transformers capable of handling primary currents on the order of 200 to 400 amperes; from 400:5 to 800:5 for current transformers capable of handling primary currents on the order of 400 to 800 amperes; from 1200:5 to 2000:5 for current transformers capable of handling primary currents on the order of 800 to 2,000 amperes; and from 3000:5 to 12,000:5 for current transformers capable of handling primary currents on the order of 2,000 to 12,000 amperes.
Primary to secondary winding ratios ideally reflect expected primary to secondary current ratios when such current transformers are in actual operation. However, current transformers of this magnitude are not linear, that is to say, the current ratio of primary current to secondary current may very well change as a function of the primary current itself. Accordingly, it is the actual primary current to secondary current ratio under load conditions which must be accurately measured to determine the operational characteristics of such transformers.
Primary current to secondary current ratio measurement is presently very difficult to obtain. Using known techniques, such measurement must occur while the transformer is out of service, that is, while the primary winding of the transformer is disconnected from any unregulated current supply. In this condition, a known source current can be applied to the current transformer and the resultant secondary current measured. A ratio between the known source current and measured secondary current can then be mathematically calculated to determine the primary to secondary current ratio. Of course, large well-regulated current sources are required to utilize this method of obtaining primary current to secondary current ratios for large current transformers.
In the alternative, an unknown current may be applied to the primary windings and an attempt may be made by two persons simultaneously to read both the primary current and the secondary current using current probes and hand-held meters. At a given instant of time, simultaneous readings are taken and the ratio between these two readings manually calculated. Obviously, such prior art methodology is cumbersome and inaccurate.
It is, accordingly, an object of the present invention to provide a portable tester which is capable of automatically, efficiently, and accurately determing the primary to secondary winding current ratio of a current transformer without the necessity of removing that transformer from active service.
In other words, it is an object of the subject invention to obtain simultaneous acquisition and display of primary and secondary currents of an in-service current transformer and automatically calculate and display the ratio of those currents.
Additional objects and advantages of the invention will be set forth in the description which follows and in part will be obvious from the description or may be learned by practice of the invention.